As is well known to those skilled in the art, it is possible to separate mixtures of liquids by various techniques including adsorption or distillation. These conventional processes, particularly distillation, are however characterized by high capital cost. In the case of distillation for example, the process requires expensive distillation towers, heaters, heat exchangers (reboilers, condensers, etc.), together with a substantial amount of auxiliary equipment typified by pumps, collection vessels, vacuum generating equipment, etc.
Such operations are characterized by high operating costs principally costs of heating and cooling--plus pumping, etc.
Furthermore the properties of the materials being separated, as is evidenced by the distillation curves, may be such that a large number of plates may be required, etc. When the charge components form an azeotrope, additional problems may be present which for example, could require that separation be effected in a series of steps (e.g. as in two towers) or by addition of extraneous materials to the system.
There are also comparable problems which are encountered in adsorption systems.
It has been found to be possible to utilize membrane systems to separate mixtures of miscible liquids by pervaporation. In this process, the charge liquid is brought into contact with a membrane film; and one component of the charge liquid preferentially permeates the membrane. The permeate is then removed as a vapor from the downstream side of the film--typically by sweeping with a carrier gas or by reducing the pressure below the vapor pressure of the permeating species.
Illustrative membranes which have been employed in prior art techniques include those set forth in the following table:
TABLE ______________________________________ Separating Layer References ______________________________________ Polyyvinyl alcohol Kuraray Co. Japanese containing glycerine Patent 81/193495 (1981) JP 58/g5522A2 (1983) Nafion brand of Cabasso and Liu perfluorosulfonic acid J. Memb. Sci. 24, 101 (1985) Sulfonated polyethylene Cabasso, Korngold, & Liu, J. Pol. Sci.: Letters, 23, 57 (-985) Fluorinated Polyether U.S. Pat. No. 4,526,948 or Carboxylic Acid Fluorides to DuPont as assignee of Resnickto Selemion AMV Wentzlaff, blend of Asahi Glass Boddeker, & Hattanbach cross-linked styrene - J. Memb. Sci. 22,333 butadiene (with quaternary (1985) ammonium residues on a polyvinyl chloride backing Cellulose triacetate Wentzlaff, Boddeker & Hattanback, J. Memb. Sci. 22, 333 (1985) Polyacrylonitrile or Neel, Aptel, & Polytetrafluoroethylene Clement Desalination grafted with N-vinyl 53, 297 (1985) pyrrolidone Cellulose Acetate Proc. of Int. Mem. and others Conf. Sept., 1986 Ottawa, p 229 Crosslinked Eur. Patent 0 096 Polyvinyl Alcohol 339 to GFT as assignee of Bruschke Poly(maleimide- Yoshikawa et al acrylonitrile) J. Pol. Sci. 22, 2159 (1984) Dextrine - Chem. Econ. Eng. isophoronediisocyanate Rev., 17, 34 (1985) ______________________________________
The cost effectiveness of a membrane is determined by the selectivity and productivity. Of the membranes commercially available, an illustrative membrane of high performance is that disclosed in European patent 0 096 339 A2 of GFT as assignee of Bruschke--published 21 Dec. 1983.
See also J. G. Prichard, Polyvinyl Alcohol, Basic Properties and Uses, Gordon and Breach Science Publishers, New York (1970) or
C. A. Finch, Polyvinyl Alcohol, Properties and Applications, John Wiley and Sons, New York (1973).
U.S. Pat. No. 4,952,318, issued Aug. 28, 1990, (and its divisional U.S. Pat. No. 5,006,576 issued Apr. 9, 1991) to M. Pasternak and T. G. Dorawala are directed to separation of dilute aqueous solutions of organic oxygen-containing components by use of, as a pervaporation membrane, an ion exchange resin in membrane form bearing a pendant acid group, which membrane has been contacted with a quaternary ammonium salt.
U.S. Pat. No. 4,798,674, issued Jan. 17, 1989, (and its divisional 4,877,529 issued Oct. 31, 1989) to M. Pasternak, C. R. Bartels, and J. Reale, Jr. is directed to the separation of methanol from products such as methyl t-butyl ether or dimethyl carbonate by use of, as a pervaporation membrane, either (i) a cross-linked polyvinyl alcohol membrane or (ii) an ion exchange membrane which has been contacted with a quaternary ammonium salt.
U.S. Pat. No. 4,960,519 issued Oct. 20, 1990, to M. Pasternak, C. R. Bartels, J. Reale, Jr., and V. Shah is directed to separation of alcohols from oxygenates by use, as a pervaporation membrane, of a blend of a polyvinyl alcohol and a polyacrylic acid on a polyacrylonitrile support layer.
U.S. Pat. No. 4,898,674 issued Feb. 6, 1990 to M. Pasternak and T. G. Dorawala is directed to changing the ratio of components in a mixture of components (typically toluene and methyl ethyl ketone) in a dewaxing solvent by use, as a pervaporation membrane, of an ion exchange polymer in membrane form which has been contacted with tetraphenyl phosphonium bromide or a salt of an alkali metal (such as potassium chloride).
It is an object of this invention to provide a separation process. Other objects will be apparent to those skilled in the art.